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MFJ-1780 

Box Fan

 Loop Antenna                                                         Instruction Manual 

4  

loop as a wheel, the nulls are in the same directions 
that the wheel's axle would run. Signals will be 
attenuated more than 10 dB if they arrive within 15 
degrees of the axis of the loop. See Figure 2. 
- In general this antenna, like most others, should be 
placed as far away from and as high above other 
objects as possible. The null (or broadside axis) of 
the loop should be placed in line with the direction 
that you do not want to transmit or receive. 

If the 

loop can not be placed more than 15 feet above 
ground

 

level the best placement arrangement will 

be with the loop placed vertically (sitting on its 
edge). Otherwise, (higher than 15 feet) it is  
possible to mount the antenna horizontally

 

NULLLING UNWANTED SIGNALS: 

 
In the vertical position, the 

Box Fan

 loop nulls can be used to reduce interference (if receiving) from 

undesired directions if the unwanted signal is coming from a fixed direction and wave angle, or to 
minimize interference (if transmitting) caused by the antenna itself to other household electronics 
equipment. 
 
Other examples of using the null can be understood if we consider a loop antenna placed horizontally 
on a second or higher floor of a building. By placing the antenna horizontally on the middle of the attic 
the null can be positioned directly below and the antenna (through the building). This placing 
arrangement helps receiving by reducing the noise pick-up from devices in the building and helps 
transmitting because energy is not coupled into the building's lossy structure. RFI in the building will 
also be reduced because the signal transmitted into the building is  
weaker. See Figure 3. 
 

VERTICAL POLARIZATION: 

 
When the loop is placed to provide vertical polarization the pattern is vertically polarized in line with 
the loop element. There will still be a large amount of horizontally polarized radiation broadside to the 
loop, especially if the ground under the loop is less than perfect or if the loop is placed some distance 
above ground. 
 
The broadside horizontal radiation that occurs in a small vertical loop is mostly above 10 degree wave 
angles and extends straight above the loop and to the opposite 10 degree elevation point. 

True vertical 

polarization occurs only in line with the loop

. As you move around the loop towards the sides, the 

pattern skews and eventually becomes completely horizontal broadside to the loop. See Figure 4. 
Unlike linear verticals, vertically polarized small loops also radiate straight up and down from the 
antenna. This high angle of radiation can be used to cover short distances by sky wave. The high angle 
horizontal radiation in a vertically polarized loop antenna occurs because the ground below the loop is 

 

Figure 2

 

Summary of Contents for Box Fan -1780

Page 1: ...NTED SIGNALS 4 Vertical Polarization 4 Horizontal Polarization 5 OUTDOOR USE 5 INDOOR USE 6 CONTROLLER BOX INSTALLATION 7 CONTROLLER BOX OPERATION 7 TESTING AND OPERATION 9 GENERAL OPERATION AND TROUB...

Page 2: ...all welded low resistance high current butterfly tuning capacitor WARNINGS Keep this antenna out of reach of adults children and animals Any contact with this antenna while transmitting will cause sev...

Page 3: ...little difference between the signal from the MFJ Box Fan Loop and the signal from a larger size none portable loop at the same height PATTERNS POLARIZATION AND LOCATION WARNING Keep this antenna out...

Page 4: ...of the attic the null can be positioned directly below and the antenna through the building This placing arrangement helps receiving by reducing the noise pick up from devices in the building and hel...

Page 5: ...ot expect the best results if you mount this loop antenna horizontally less than 15 feet above ground Since ground wave signals only propagate well along the earth when they are vertically polarized a...

Page 6: ...ace is available The MFJ 1780 is a portable antenna with an easy carrying handle you can take it with you anywhere hotel camp sites traveling etc You can place the antenna on a picnic table wooden tab...

Page 7: ...You must only use the MFJ 1312B supply or batteries to operate this unit If you chose to use an other power supply to power this unit the power supply must not have a common ground with anything else...

Page 8: ...xial plug with the center conductor positive The MFJ 1312B power supply will supply the correct power to the controller box There is also an internal battery connection that uses two 9 volt batteries...

Page 9: ...Y FURTHER MAKE SURE YOU UNDERSTAND ALL WARNINGS BEFORE YOU ATTEMPT TO TEST OR OPERATE THIS ANTENNA 1 With power disconnected set the group FAST TUNE switches in the OFF position out 2 Connect the loop...

Page 10: ...e capacitor back to the tuning position with the lowest SWR Note The tuning of this antenna is very sharp due its high Q and narrow bandwidth few KHz Watch you meter carefully while tuning Now you hav...

Page 11: ...ight as the loop moves to the highest frequency range After the LED goes out release the Fast Tune UP button 2 Select the proper wattmeter range for the power you intend to use in tuning the loop then...

Page 12: ...ground If you push a Tune button and the Tune LED stays on for more than 40 seconds Check for moisture in the feedline or antenna If you press a Tune button and an LED does not light Check to see if...

Page 13: ...f the continental U S A 601 323 5869 You will be best served if you have your unit manual and all information on your station handy so you can answer any questions the technicians may ask You can also...

Page 14: ...is available by calling 601 323 5869 5 This warranty does not apply to kits sold by or manufactured by MFJ Enterprises Inc 6 Wired and tested PC board products are covered by this warranty provided o...

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